Spin torque MRAM based on Co, Ir synthetic antiferromagnetic multilayer

ABSTRACT

Magnetic memory devices having an antiferromagnetic reference layer based on Co and Ir are provided. In one aspect, a magnetic memory device includes a reference magnetic layer having multiple Co-containing layers oriented in a stack, wherein adjacent Co-containing layers in the stack are separated by an Ir-containing layer such that the adjacent Co-containing layers in the stack are anti-parallel coupled by the Ir-containing layer therebetween; and a free magnetic layer separated from the reference magnetic layer by a barrier layer. A method of writing data to a magnetic random access memory device having at least one of the present magnetic memory cells is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 14/561,690filed on Dec. 5, 2014, the contents of which are incorporated byreference as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to magnetic memory devices and moreparticularly, to magnetic memory devices having an antiferromagneticreference layer based on cobalt (Co) and iridium (Ir).

BACKGROUND OF THE INVENTION

Semiconductor devices, such as magnetic random access memory (MRAM)devices, use magnetic memory cells to store information. Information isstored in the magnetic memory cells as an orientation of themagnetization of a free layer in the magnetic memory cell as compared toan orientation of the magnetization of a fixed (e.g., reference) layerin the magnetic memory cell. The free layer and the fixed layer,separated by a tunnel barrier, form a magnetic tunnel junction.

The magnetization of the free layer can be oriented parallel oranti-parallel relative to the fixed layer, representing either a logic“1” or a logic “0.” The orientation of the magnetization of a givenlayer (fixed or free) may be represented by an arrow pointing either tothe left or to the right. When the magnetic memory cell is sitting in azero applied magnetic field, the magnetization of the magnetic memorycell is stable, pointing either left or right. Driving a current throughthe magnetic tunnel junction can cause the magnetization of the freelayer to switch due to spin transfer torque from left to right, and viceversa, to write information to the magnetic memory cell. See, forexample, Worledge et al., “Spin torque switching of perpendicularTa|CoFeB|MgO-based magnetic tunnel junctions,” Applied Physics Letters98, 022501 (January 2011) (hereinafter “Worledge”), the contents ofwhich are incorporated by reference as if fully set forth herein. Asdescribed in Worledge, with spin torque MRAM devices perpendicularmagnetic anisotropy greatly reduces the switching voltage.

To achieve a reliable reading and writing on spin torque MRAM devices, astable reference layer is a key prerequisite. The reference layer has tobe rigid both under magnetic field and under the application of current.Therefore, an ideal reference layer has to own a large coercive fieldand a strong anisotropy energy density. Meanwhile, in order to operatethe MRAM device under zero or small external magnetic field, thereference layer has to induce a minimal dipole field on the free layer.This means that the magnetic moment of the reference layer has to belargely internally canceled. For instance, multilayer structures basedon cobalt (Co)/platinum (Pt) (or Co/palladium (Pd), Co/nickel (Ni),etc.) have been proposed for use as a reference layer in MRAM devices.However, the [Co/Pt]_(N) multilayers are ferromagnetically aligned andas such the magnetic moments of the neighboring Co layers are parallelcoupled. Thus, a dipole moment will build up and, without a largeexternal offset field, this type of reference layer will undesirablyexert a large dipole magnetic field onto the free layer.

To overcome this issue, it was proposed in Worledge to insert ruthenium(Ru) between the Co/Pt layers, i.e., resulting in a[Co/Pt]_(N)/Ru/[Co/Pt]_(N) structure. By inserting Ru, the magneticmoment of the [Co/Pt]_(N) multilayer on top of the Ru will point in theopposite direction of the magnetic moment of the [Co/Pt)]_(N) multilayerbelow the Ru, cancelling each other out, and thus solving the issue of alarge dipole moment. However, there are some notable drawbacks to use ofRu. First, the perpendicular magnetic anisotropy is usually compromisedbecause of the introduction of Ru. Second, Ru diffuses throughout thelayers during high temperature anneals compromising function of thedevice. Moreover, reference layers made from Co/Pt usually have poorperformance in thermal stability. The perpendicular anisotropy and themagnetoresistance of the magnetic tunnel junction will degrade afterhigh temperature annealing because of the diffusion of Pt (or Ni, Pd) inthose materials.

Therefore, a stable and low dipole field reference layer forperpendicular magnetic anisotropy spin torque MRAM devices would bedesirable.

SUMMARY OF THE INVENTION

The present invention provides magnetic memory devices having anantiferromagnetic reference layer based on cobalt (Co) and iridium (Ir).In one aspect of the invention, a magnetic memory cell is provided. Themagnetic memory cell includes a reference magnetic layer having multipleCo-containing layers oriented in a stack, wherein adjacent Co-containinglayers in the stack are separated by an Ir-containing layer such thatthe adjacent Co-containing layers in the stack are anti-parallel coupledby the Ir-containing layer therebetween; and a free magnetic layerseparated from the reference magnetic layer by a barrier layer.

In another aspect of the invention, a method of writing data to amagnetic random access memory device having a plurality of word linesoriented orthogonal to a plurality of bit lines, and a plurality ofmagnetic memory cells configured in an array between the word lines andbit lines is provided. The method includes the steps of: providing aword line current to a given one of the word lines to select all of themagnetic memory cells along the given word line, providing a bit linecurrent to each of the bit lines corresponding to the selected magneticmemory cells; removing the word line current; and removing the bit linecurrent. At least one of the selected magnetic memory cells includes: areference magnetic layer having multiple Co-containing layers orientedin a stack, wherein adjacent Co-containing layers in the stack areseparated by an Ir-containing layer such that the adjacent Co-containinglayers in the stack are anti-parallel coupled by the Ir-containing layertherebetween; and a free magnetic layer separated from the referencemagnetic layer by a barrier layer.

In yet another aspect of the invention, another magnetic memory cell isprovided. The magnetic memory cell includes: a reference magnetic layerhaving multiple subsystems of layers oriented in a stack, wherein eachof the subsystems of layers includes a first Co-containing layerseparated from a second Co-containing layer by a platinum(Pt)-containing layer such that the first Co-containing layer and thesecond Co-containing layer are parallel coupled by the Pt-containinglayer therebetween, and wherein adjacent subsystems of layers in thestack are separated by an Ir-containing layer such that the adjacentsubsystems of layers in the stack are anti-parallel coupled by theIr-containing layer therebetween; and a free magnetic layer separatedfrom the reference magnetic layer by a barrier layer.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating a reference layerhaving been formed on a (e.g., seeding layer) substrate, wherein thereference layer includes a stack of multiple cobalt (Co)-containinglayers separated by iridium (Ir)-containing layers, and wherein athickness of the Co-containing layers varies based on their locationwithin the stack according to an embodiment of the present invention;

FIG. 1A is a cross-sectional diagram illustrating one exemplaryconfiguration of the reference layer where for each (anti-parallelcoupled) pair of the Co-containing layers in the stack, a same thicknessis employed for the lower-in-the-stack/thicker Co-containing layer ineach pair, and a same thickness is employed for thehigher-in-the-stack/thinner Co-containing layer in each pair accordingto an embodiment of the present invention;

FIG. 2 is a cross-sectional diagram illustrating an interfacial layer(e.g., Ta|CoFeB) having been formed on a side of the reference layeropposite the substrate according to an embodiment of the presentinvention;

FIG. 3 is a cross-sectional diagram illustrating a tunneling barrierlayer having been formed on a side of the interfacial layer opposite thereference layer according to an embodiment of the present invention;

FIG. 4 is a cross-sectional diagram illustrating a free layer havingbeen formed on a side of the tunneling barrier layer opposite theinterfacial layer according to an embodiment of the present invention;

FIG. 5 is a cross-sectional diagram illustrating a capping layer havingbeen formed on a side of the free layer opposite the tunneling barrierlayer according to an embodiment of the present invention;

FIG. 6 is a cross-sectional diagram illustrating a reference layerhaving been formed on a (e.g., seeding layer) substrate, wherein thereference layer includes a stack of multiple Co-/Pt-/Co-containing layersubsystems separated by Ir-containing layers according to an embodimentof the present invention;

FIG. 7 is a cross-sectional diagram illustrating an interfacial layerhaving been formed on a side of the reference layer opposite thesubstrate, a tunneling barrier layer having been formed on a side of theinterfacial layer opposite the reference layer, a free layer having beenformed on a side of the tunneling barrier layer opposite the interfaciallayer and a capping layer having been formed on a side of the free layeropposite the tunneling barrier layer according to an embodiment of thepresent invention;

FIG. 8 is a diagram illustrating an exemplary magnetic memory cell arrayaccording to an embodiment of the present invention;

FIG. 9 is a diagram illustrating an exemplary methodology for writingdata to a magnetic memory cell array according to an embodiment of thepresent invention;

FIG. 10A is a diagram illustrating antiferromagnetic coupling ofCo-containing layers via an Ir-containing layer having a thickness of4.5 angstroms according to an embodiment of the present invention;

FIG. 10B is a diagram illustrating antiferromagnetic coupling ofCo-containing layers via an Ir-containing layer having a thickness of5.0 angstroms according to an embodiment of the present invention;

FIG. 11A is a diagram illustrating perpendicular magnetic anisotropy(perpendicular loop) of a pair of Co-containing layers having athickness t1=7.5 and t2=5 according to an embodiment of the presentinvention; and

FIG. 11B is a diagram illustrating perpendicular magnetic anisotropy(inplane loop) of a pair of Co-containing layers having a thicknesst1=7.5 and t2=5 according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are perpendicular magnetic anisotropy spin torque MRAMdevices (also referred to herein as “perpendicular spin torque MRAMdevices”) having magnetic memory cells with multilayer cobalt (Co) andiridium (Ir)-based reference layers. As described above, magnetic memorycells generally include a free layer and a fixed (or reference) layerseparated by a tunnel barrier, forming a magnetic tunnel junction. Inthis case, the reference layer includes a synthetic antiferromagneticmultilayer having alternating Co- and Ir-containing layers. The term“synthetic” is used to differentiate the present antiferromagneticmultilayer reference layer from real, natural antiferromagneticmaterials (i.e., in real, natural antiferromagnetic materials theneighboring magnetic atoms are anti-parallel aligned and thus one wouldnot need to create the kind of multilayer repeat structure providedherein).

Like ruthenium (Ru), Ir effects anti-parallel coupling of theCo-containing layers. Therefore, as provided above, the net result isthat the magnetic moments of the Co-containing layers cancel each otherout resulting in a low dipole field (i.e., less than about 300 oersted(Oe)). However unlike Ru, Ir advantageously strengthens theperpendicular magnetic anisotropy thus making the multilayer referencelayer stack more stable. Without being bound by any particular theory,it is thought that even though Ru and Ir are similar elements therelatively larger atomic number of Ir affords a greater perpendicularmagnetic anisotropy. As a result, with the present device configurationsit is desirable to use multiple Ir-containing layers in the referencelayer stack—see below. By comparison, since Ru generally compromises theperpendicular magnetic anisotropy (see above), reference layers soconfigured to include Ru typically limit the Ru to a single layer in thestack.

An exemplary methodology for forming a magnetic memory cell according tothe present techniques is now described by way of reference to FIGS.1-5. As shown in FIG. 1, the process begins with the formation of thereference magnetic layer (or simply “reference layer”) which in thisexemplary embodiment includes multiple Co-containing layers oriented ina stack. As will be described in detail below, an Ir-containing layer ispresent between each of the adjacent Co-containing layers in the stack.

In the exemplary embodiment shown, this multilayer reference layer(depicted generally as reference layer 104) is formed on a substrate102. Preferably, the substrate 102 is configured to serve as a seedinglayer which provides a proper texture for building the reference layerconstituent films. For instance, employing a (111) orientation in thecrystal structure of the Co- and Ir-containing layers has been found toenhance the perpendicular magnetic anisotropy of the reference layerstack. Suitable seeding layer materials which induce a (111)crystallographic orientation of the Co- and Ir-containing layersinclude, but are not limited to, platinum (Pt), Ru, and Ir. According toan exemplary embodiment, the seeding layer has a thickness of greaterthan about 5 nanometers (nm), for example, from about 5 nm to about 50nm, and ranges therebetween. Of these seeding materials, Pt and Irshould have a face centered cubic (FCC) structure, while Ru is usuallyhexagonal close-packed (HCP). FCC 111 and HCP 001 orientations aresimilar in that they both give a close compact structure and can bothgive textures for the FCC 111 layers grown on top thereof (the Co- andIr-containing multilayer in this case). Both Co and Ir have a FCCstructure. The 111 orientation is the closed pack direction, meaningthat the overall energy will be the lowest if the film grows along thisdirection. Thus, if the Co- and Ir-containing layers start with thecorrect texture, this orientation will be carried over to the wholethickness. The fact that Ir has the smallest lattice mismatch with Ir/Comakes it particularly well suited for use as a seeding layer in thiscase, and has been found by way of experimentation to give rise to thestrongest perpendicular magnetic anisotropy in the present magneticmemory cell. See below.

Next, as shown in FIG. 1, a multilayer stack of alternating Co- andIr-containing layers are formed on the substrate 102. This multilayerstack will form the reference layer 104 of the present magnetic memorycell.

As shown in FIG. 1, and as described below, one of the Ir-containinglayers will be present between each of the adjacent Co-containing layersin the stack. These Ir-containing layers serve to anti-parallel couplethe Co-containing layers. By anti-parallel coupling (also referred toherein as “anti-ferromagnetic coupling”) the Co-containing layers in thestack the net dipole moment is small. Thus, the reference layer inducesat most a minimal dipole field on the free layer. Further, since thepresence of the Ir-containing layers in the stack actually strengthensthe perpendicular magnetic anisotropy (see above) here one can afford tohave as many Ir-containing layers as possible. By comparison, the Ruemployed in conventional configurations compromises the perpendicularmagnetic anisotropy, and thus reference layers so configured to includeRu typically limit the Ru to a single layer in the stack.

Any suitable process known in the art may be employed to deposit/formthe constituent layers of the reference layer stack. By way of exampleonly, each of the layers of the present magnetic memory cell may beformed using a sputtering deposition technique, such as direct current(DC) sputtering. It would be within the capabilities of one skilled inthe art to configure a sputtering or other standard deposition processto produce layers of the present materials having the particularcompositions, thicknesses, etc. described herein.

Referring to FIG. 1, the process for forming the reference layer 104begins by forming a first Ir-containing layer on the substrate 102. Asprovided above, the substrate 102 is preferably configured to serve(i.e., based on its composition, thickness, etc.) as a seeding layer togrow Ir- and Co-containing layers thereon having a (111)crystallographic orientation. According to an exemplary embodiment, theIr-containing layer is formed on the substrate 102 having a thickness offrom about 4 angstroms (Å) to about 9 Å, and ranges therebetween, forexample, from about 4.5 Å to about 5 Å, and ranges therebetween. Asdescribed above, the Ir-containing layers serve to anti-parallel couplethe Co-containing layers in the stack thereby minimizing the net dipolemoment. Thus, according to an exemplary embodiment, an Ir-containinglayer thickness is chosen that gives the best anti-parallel coupling(based, for example, on electrical characteristics of test samples—seebelow) between Co-containing layers (for example a thickness within theranges provided above), and then that same thickness is employed foreach of the Ir-containing layers in the stack.

Next, as shown in FIG. 1, a first Co-containing layer Co1 is formed on aside of the first Ir-containing layer Ir1 opposite the substrate 102, asecond Ir-containing layer Ir2 is formed on a side of the firstCo-containing layer Co1 opposite the first Ir-containing layer Ir1, anda second Co-containing layer Co2 is then formed on a side of the secondIr-containing layer Ir2 opposite the first Co-containing layer Co1, etc.For ease and clarity of description, each of the Ir- and Co-containinglayers in the stack are given the designation Ir1, Co1, Ir2, Co2, . . ., IrX, CoX, IrY, CoY which correspond to the first (Ir- andCo-containing layers), the second (Ir- and Co-containing layers), X^(th)(Ir- and Co-containing layers), and Y^(th) (Ir- and Co-containinglayers) being formed. Each pair of Co-containing layers (i.e., Co1/Co2,. . . , CoX/CoY) are anti-parallel coupled via the Ir-containing layertherebetween. As shown in the figures, the stack will end up having Nnumber of pairs of the Co-containing layers. Having an even number ofCo-containing layers (i.e., the Co-containing layers are present in thestack as (anti-parallel coupled) pairs) ensures that the magneticmoments of Co-containing layers cancel each other out. According to anexemplary embodiment, N is from 1 to 5, and ranges therebetween, e.g., Nis 3. It is thus notable that while FIG. 1 shows multiple anti-parallelcoupled pairs, this is done to illustrate how the repeating pairs wouldbe oriented in the stack, and the stack could in some exemplaryembodiments include a single pair of anti-parallel coupled Co-containinglayers (i.e., where N=1). Further, according to the exemplary embodimentshown illustrated in FIG. 1, there are an equal number of Co-containingand Ir-containing layers in the stack. This is however merely anexample, and configurations are anticipated herein where differingnumbers of Co-containing layers and Ir-containing layers are present inthe reference layer stack.

When determining the appropriate thickness of a given one of theCo-containing layers, it is preferable to take into account the locationof that Co-containing layer in the stack. For instance, it has beenfound that, for each anti-parallel coupled pair of Co-containing layersin the stack, the Co-containing layer in the pair closer in the stack tothe free layer will have a greater (magnetic moment) influence on thefree layer than the Co-containing layer in the pair farther away in thestack from the free layer. To use the structure in FIG. 1 as an example,the free layer will be formed on top of the reference layer 104. Thus,in the example shown, Co1 and Co2 constitute an anti-parallel coupled(via Ir2) pair of Co-containing layers. Of that pair, Co2 will berelatively closer to the free layer than is Co1. Thus, to compensate forthat greater influence due to closer proximity, the Co2 layer ispreferably thinner than the Co1 layer. The same applies, for example, toanti-parallel coupled pair of CoX and CoY (coupled via IrY). What isimportant is that so long as t1−t2>0, . . . , tX−tY>0, one can ensurethat the overall moment will favor the layers that are relativelyfurther away from the free layer. Further, since the Co-containing pairsare anti-parallel coupled repeats throughout the stack, then one onlyreally needs to address the thickness of the Co-containing layers in agiven anti-parallel coupled pair relative to one another. For instance,the thickness of Co2 need only be evaluated relative to the thickness ofCo1, so as to ensure Co2 is thinner than Co1 for the above-statedreasons. Similarly, the thickness of CoY should be evaluated relative tothe thickness of CoX, so as to ensure CoY is thinner than CoX. Toillustrate this point, take for example, the case where N=2, and thereference layer stack includes only the four Co-containing layers—Co1,Co2, CoX, and CoY (and the corresponding Ir-containing layers). Co1 andCoX would have magnetic moments pointing in the same direction, as wouldCo2 and CoY. Thus while the thickness of Co2 relative to Co1 and thethickness of CoY relative to CoX is important, the thickness of Co2relative to CoX is not an important consideration. By way of exampleonly, an illustrative non-limiting example is shown in FIG. 1A whereinthe same thickness is used for each of the lower-in-the-stack/thickerCo-containing layers in each anti-parallel coupled pair, e.g., t1 (ofCo1) is equal to tX (of CoX). Similarly, the same thickness is used foreach of the higher-in-the-stack/thinner Co-containing layers in eachanti-parallel coupled pair, e.g., t2 (of Co2) is equal to tY (of CoY).It is by no means required that the reference layer stack have thisparticular configuration. However, FIG. 1A provides an example of areference layer stack configuration that meets the above Co-containinglayer thickness requirements.

Thus, according to an exemplary embodiment the thicknesses of theCo-containing layers are varied depending on the location of the layerin the stack. In general, in this example the Co-containing layer ineach anti-parallel coupled pair of Co-containing layers that is(relatively) closer to the free layer (i.e., higher in the stack) willbe thinner than the Co-containing layer in the anti-parallel coupledpair (relatively) farther away from the free layer (i.e., lower in thestack). To implement this configuration, one can simply decrease thethickness of the Co-containing layer formed when building eachanti-parallel coupled pair of Co-containing layers. See, for example,FIG. 1 where Co-containing layers Co1 and Co2 have thicknesses t1 andt2, respectively. Thus, during fabrication, Co2 can be formed having athickness that is less than Co1 (t2<W. Similarly, Co-containing layersCoX and CoY have thicknesses tX and tY, respectively. Thus, duringfabrication, CoY can be formed having a thickness that is less than CoX(tY<tX).

By way of example only, the first Co-containing layer (in a pair) can bedeposited to a given thickness t1. The thickness of the next layer inthe pair (and higher up in the stack) can then be reduced by from about20% to about 50%. According to an exemplary embodiment, t1 is from about5 Å to about 12.5 Å, and ranges therebetween. Thus, to give a simpleexample using the Co1-Co2 pair from FIG. 1, if the thickness t1 of Co1is 5 Å, then a thickness t2 of Co2 can be 2.5 Å (a thickness reductionof 50%).

Next, as shown in FIG. 2, an interfacial layer 202 is formed on a sideof the reference layer 104 opposite the substrate 102. As is known inthe art, the use of an interfacial layer increases magnetoresistance inmagnetic tunnel junctions. According to an exemplary embodiment, theinterfacial layer is formed from cobalt-iron-boron (CoFeB) with atantalum (Ta) underlayer. The use of Ta/CoFeB interfacial layers in amagnetic tunnel junction is described, for example, in Worledge and inSinha et al., “Enhanced interface perpendicular magnetic anisotropy inTa|CoFeB|MgO using nitrogen doped Ta underlayers,” arXiv:1305.6660 (June2013) (hereinafter “Sinha”), the contents of which are incorporated byreference as if fully set forth herein. For instance, as described inSinha a film stack was prepared using magnetron sputtering containingnitrogen doped Ta(TaN)|Co₂₀Fe₆₀B₂₀. This same Ta|CoFeB can beimplemented in accordance with the present techniques as the interfaciallayer 202. See, for example, FIG. 2.

A tunneling barrier layer 302 is then formed on a side of theinterfacial layer 202 opposite the reference layer 104. See FIG. 3.According to an exemplary embodiment, the tunneling barrier layer 302 isformed from magnesium oxide (MgO). The use of a MgO tunneling barrier isdescribed, for example, in Worledge and Sinha. As provided above, theMgO tunneling barrier may be formed using a sputtering process.

Next, as shown in FIG. 4, a free magnetic layer (or simply “free layer”)402 is formed on a side of the tunneling barrier layer 302 opposite theinterfacial layer 202. According to an exemplary embodiment, free layer402 is formed from CoFeB deposited using a sputtering process. See, forexample, Worledge and Sinha. This free layer 402 and the above-describedreference layer 104 which are separated by the tunneling barrier layer302 form a magnetic tunnel junction. As described above, information isstored in the tunnel junction magnetic memory cell as an orientation ofthe magnetization of the free layer 402 as compared to an orientation ofthe magnetization of the reference layer 104, which according to thepresent techniques can be switched by driving a current through themagnetic tunnel junction—see below.

Finally, to complete the magnetic memory cell, a capping layer 502 isformed on a side of the free layer 402 opposite the tunneling barrierlayer 302. According to an exemplary embodiment, the capping layer 502is formed from tantalum (Ta) or a combination of Ru and Ta (i.e., a Talayer and a Ru layer on the Ta layer) using sputtering. See, forexample, U.S. Pat. No. 8,378,330 issued to Horng et al., entitled“Capping layer for a magnetic tunnel junction device to enhance dR/R anda method of making the same,” the contents of which are incorporated byreference as if fully set forth herein.

It is notable that other configurations of the reference layer for usein accordance with the present techniques are possible. By way ofexample only, the multiple Co-containing layers in the reference layerstack as described above may be replaced with a subsystem of layerswhich contain alternating Co- and Pt-containing layers. For example,according to an alternative exemplary embodiment the present referencelayer instead includes multiple subsystems oriented in a stack, whereineach of the subsystems includes two Co-containing layers separated by aPt-containing layer, and wherein an Ir-containing layer is presentbetween each adjacent subsystem in the stack. In this case, theCo-containing layers within each of the subsystems are parallel coupledvia the Pt-containing layer, while the subsystems are anti-parallelcoupled via the Ir-containing layer. This alternative exemplaryembodiment is now described in detail by way of reference to FIGS. 6 and7.

As with the first embodiment presented above, the process begins withthe formation of the multilayer reference layer stack. See FIG. 6. Inthe exemplary embodiment shown, this multilayer reference layer(depicted generally as reference layer 604) is formed on a substrate602. As provided above, the substrate 602 is preferably configured toserve as a seeding layer which provides a proper texture for buildingthe reference layer constituent Co-, Pt-, and Ir-containing films havinga desired (111) crystallographic orientation so as to enhance theperpendicular magnetic anisotropy of the reference layer stack. Suitableseeding layer materials include, but are not limited to, Pt, Ru, and Ir.According to an exemplary embodiment, the seeding layer has a thicknessof greater than about 5 nm, for example, from about 5 nm to about 50 nm,and ranges therebetween.

Next, as shown in FIG. 6, a multilayer stack of alternatingCo-/Pt-/Co-(subsystem) and Ir-containing layers are next formed on thesubstrate 602. This multilayer stack will form the reference layer 604of the present magnetic memory cell. The same techniques as describedabove also apply here except that instead of forming multipleCo-containing layers, multiple subsystems of Co-/Pt-/Co-containing(parallel-coupled) layers are formed. Otherwise the processes are thesame.

As shown in FIG. 6, one of the Ir-containing layers will be presentbetween each of the adjacent Co-/Pt-/Co-containing layer subsystems inthe stack. These Ir-containing layers serve to anti-parallel couple theCo-/Pt-/Co-containing layer subsystems. By anti-parallel coupling theCo-/Pt-/Co-containing layer subsystems in the stack the net dipolemoment is small. Thus, the reference layer induces at most a minimaldipole field on the free layer. Further, since the presence of theIr-containing layers in the stack actually strengthens the perpendicularmagnetic anisotropy (see above) here one can afford to have as manyIr-containing layers as possible. By comparison, the Ru employed inconventional configurations compromises the perpendicular magneticanisotropy, and thus reference layers so configured to include Rutypically limit the Ru to a single layer in the stack.

Any suitable process known in the art may be employed to deposit/formthe constituent layers of the reference layer stack. By way of exampleonly, each of the layers of the present magnetic memory cell may beformed using a sputtering deposition technique, such as direct current(DC) sputtering. It would be within the capabilities of one skilled inthe art to configure a sputtering or other standard deposition processto produce layers of the present materials having the particularcompositions, thicknesses, etc. described herein.

Referring to FIG. 6, the process for forming the reference layer 604begins by forming a first Ir-containing layer on the substrate 602. Asprovided above, the substrate 602 is preferably configured to serve(i.e., based on its composition, thickness, etc.) as a seeding layer togrow Ir-, Pt- and Co-containing layers thereon having a (111)crystallographic orientation. According to an exemplary embodiment, theIr-containing layer is formed on the substrate 602 having a thickness offrom about 4 Å to about 9 Å, and ranges therebetween, for example, fromabout 4.5 Å to about 5 Å, and ranges therebetween. As described above,the Ir-containing layers serve to anti-parallel couple the Co-containinglayers in the stack thereby minimizing the net dipole moment. Thus,according to an exemplary embodiment, an Ir-containing layer thicknessis chosen that gives the best anti-parallel coupling (based, forexample, on electrical characteristics of test samples—see below)between Co-/Pt-/Co-containing layer subsystems (for example a thicknesswithin the ranges provided above), and then that same thickness isemployed for each of the Ir-containing layers in the stack.

Next, as shown in FIG. 6, a first Co-/Pt-/Co-containing layer subsystemSubsystem1 is formed on a side of the first Ir-containing layer Ir1opposite the substrate 602, a second Ir-containing layer Ir2 is formedon a side of the first Co-/Pt-/Co-containing layer subsystem Subsystem1opposite the first Ir-containing layer Ir1, and a secondCo-/Pt-/Co-containing layer subsystem Subsystem2 is then formed on aside of the second Ir-containing layer Ir2 opposite the firstCo-/Pt-/Co-containing layer subsystem Subsystem1, etc. In thisconfiguration, the first Co-/Pt-/Co-containing layer subsystemSubsystem1 forms an anti-parallel coupled pair with the secondCo-/Pt-/Co-containing layer subsystem Subsystem2 (via the secondIr-containing layer Ir2). Similarly, the X^(th) Co-/Pt-/Co-containinglayer subsystem SubsystemX forms an anti-parallel coupled pair with theY^(th) Co-/Pt-/Co-containing layer subsystem SubsystemY (via the Y^(th)Ir-containing layer IrY), etc. Each of the Co-/Pt-/Co-containing layersubsystems in the stack can be formed by i) depositing a firstCo-containing layer (in this case on a side of the first Ir-containinglayer opposite the substrate 602), ii) depositing a Pt-containing layeron the first Co-containing layer (in this case on a side of the firstCo-containing layer opposite the first Ir-containing layer)—whichparallel couples the subsystems directly above and below it, and iii)depositing a second Co-containing layer on a side of the Pt-containinglayer opposite the first Co-containing layer. For ease and clarity ofdescription, each of the Ir-containing layers and the subsystems in thestack are given the designation Ir1, Subsystem1, Ir2, Subsystem2, . . ., IrX, SubsystemX, IrY, SubsystemY etc. which correspond to the first(Ir-containing layer and Subsystem), the second (Ir-containing layer andSubsystem), X^(th) (Ir-containing layer and Subsystem), the Y^(th)(Ir-containing layer and Subsystem) being formed. Further, within eachof the subsystems, the constituent Co- and Pt-containing layers aregiven the designation Co1a, Pt1, Co1b, etc. which corresponds to thefirst (a) Co-containing layer, the Pt-containing layer, and the second(b) Co-containing layer being formed in the first subsystem1, etc.

As highlighted above, each pair of Co-/Pt-/Co-containing layersubsystems (i.e., Subsystem1/Subsystem2, . . . , SubsystemX/SubsystemY)are anti-parallel coupled via the Ir-containing layer therebetween. Asshown in the figures, the stack will end up having N number of pairs ofthe Co-/Pt-/Co-containing layer subsystems. Having an even number ofCo-/Pt-/Co-containing layer subsystems (i.e., the Co-/Pt-/Co-containinglayer subsystems are present in the stack as (anti-parallel coupled)pairs) ensures that the magnetic moments of Co-/Pt-/Co-containing layersubsystems cancel each other out. According to an exemplary embodiment,N is from 1 to 5, and ranges therebetween, e.g., N is 3. It is thusnotable that while FIG. 6 shows multiple anti-parallel coupled pairs,this is done to illustrate how the repeating pairs would be oriented inthe stack, and the stack could in some exemplary embodiments include asingle pair of anti-parallel coupled subsystems (i.e., where N=1).Further, according to the exemplary embodiment shown illustrated in FIG.6, there are an equal number of Co-/Pt-/Co-containing layer subsystemsand Ir-containing layers in the stack. This is however merely anexample, and configurations are anticipated herein where differingnumbers of Co-/Pt-/Co-containing layer subsystems and Ir-containinglayers are present in the reference layer stack.

When determining the appropriate thickness of a given one of theCo-/Pt-/Co-containing layer subsystems (based on the thickness of theconstituent layers in the given subsystem), in the same manner asdescribed above it is preferable to take into account the location ofthat Co-/Pt-/Co-containing layer subsystems in the stack. For instance,it has been found that, for each anti-parallel coupled pair ofCo-/Pt-/Co-containing layer subsystems in the stack, the subsystem inthe pair closer in the stack to the free layer will have a greater(magnetic moment) influence on the free layer than the subsystem in thepair farther away in the stack from the free layer. To use the structurein FIG. 6 as an example, the free layer will be formed on top of thereference layer 604. Thus, in the example shown, Subsystem1 andSubsystem2 constitute an anti-parallel coupled (via Ir2) pair ofCo-/Pt-/Co-containing layer subsystems. Of that pair, Subsystem2 will berelatively closer to the free layer than is Subsystem1. Thus, tocompensate for that greater influence due to closer proximity, theSubsystem2 is preferably thinner than the Subsystem1. The same applies,for example, to the anti-parallel coupled pair of SubsystemX andSubsystemY (coupled via IrY). What is important is that so long ast1−t2>0, . . . , tX−tY>0, one can ensure that the overall moment willfavor the layers that are relatively further away from the free layer.Further, since the subsystem pairs are anti-parallel coupled repeatsthroughout the stack, then one only really needs to address thethickness of the subsystems in a given anti-parallel coupled pairrelative to one another. For instance, the thickness of Subsystem2 needonly be evaluated relative to the thickness of Subsystem1, so as toensure Subsystem2 is thinner than Subsystem1 for the above-statedreasons. Similarly, the thickness of SubsystemY should be evaluatedrelative to the thickness of SubsystemX, so as to ensure SubsystemY isthinner than SubsystemX. To illustrate this point, take for example, thecase where N=2, and the reference layer stack includes only the foursubsystems—Subsystem1, Subsystem2, SubsystemX, and SubsystemY (and thecorresponding Ir-containing layers). Subsystem1 and SubsystemX wouldhave magnetic moments pointing in the same direction, as wouldSubsystem2 and SubsystemY. Thus while the thickness of Subsystem2relative to Subsystem1 and the thickness of SubsystemY relative toSubsystemX is important, the thickness of Subsystem2 relative toSubsystemX is not an important consideration.

Thus, according to an exemplary embodiment the thicknesses of theCo-/Pt-/Co-containing layer subsystems are varied depending on thelocation of the subsystem in the stack. In general, in this example thesubsystem in each anti-parallel coupled pair of subsystems that is(relatively) closer to the free layer (i.e., higher in the stack) willbe thinner than the subsystem in the anti-parallel coupled pair(relatively) farther away from the free layer (i.e., lower in thestack). To implement this configuration, one can simply decrease thethickness of the Co-/Pt-/Co-containing layer subsystem formed whenbuilding each anti-parallel coupled pair of subsystems. See, forexample, FIG. 6 where Subsystem1 and Subsystem2 have thicknesses t1 andt2, respectively. Thus, during fabrication, Subsystem2 can be formedhaving a thickness that is less than Subsystem1 (t2<t1). Similarly,SubsystemX and SubsystemY have thicknesses tX and tY, respectively.Thus, during fabrication, SubsystemY can be formed having a thicknessthat is less than SubsystemX (tY<tX).

By way of example only, the first Co-/Pt-/Co-containing layer subsystem(in a pair) can be deposited to a given thickness t1. As provided above,the thickness of a given one of the subsystems is dependent on thethickness of the constituent Co- and Pt-containing layers. Forsimplicity, it is assumed herein that each of the constituent Co- andPt-containing layers in a given one of the subsystems has the samethickness. Thus, each of the constituent Co- and Pt-containing layers ina given one of the subsystems can be deposited to a thickness that is ⅓the desired thickness of the corresponding subsystem (since it is threeof these layers, i.e., Coa, Pt, Cob, that make up each subsystem). Thethickness of the next subsystem in the pair (and higher up in the stack)can then be reduced by from about 20% to about 50%. According to anexemplary embodiment, t1 is from about 7.5 Å to about 20 Å, and rangestherebetween. Thus, to give a simple example using theSubsystem1-Subsystem2 pair from FIG. 6, if the thickness t1 ofSubsystem1 is 8 Å, then a thickness t2 of Subsystem2 can be 4.0 Å (athickness reduction of 50%).

Reference can be made to FIG. 1A (described above) for a depiction of anexemplary case meeting these thickness requirements where the samethickness is used for each of the lower-in-the-stack/thickerCo-containing layers in each anti-parallel coupled pair, e.g., t1 (ofCo1) is equal to tX (of CoX). Similarly, the same thickness is used foreach of the higher-in-the-stack/thinner Co-containing layers in eachanti-parallel coupled pair, e.g., t2 (of Co2) is equal to tY (of CoY).In this particular case, each Co1, Co2, . . . , CoX, CoY layer shown inFIG. 1A would represent one Co-/Pt-/Co-containing layer subsystem in thereference layer stack.

The remainder of the process is the same as that described inconjunction with the description of FIGS. 1-5 above, and thus isdepicted in a single figure, FIG. 7. As shown in FIG. 7, an interfaciallayer 702 is formed on a side of the reference layer 604 opposite thesubstrate 602. According to an exemplary embodiment, the interfaciallayer is formed from CoFeB with a Ta underlayer, i.e., Ta|CoFeB. See,for example, FIG. 7. A tunneling barrier layer 704 is then formed on aside of the interfacial layer 702 opposite the reference layer 604.According to an exemplary embodiment, the tunneling barrier layer 704 isformed from MgO.

A free layer 706 is formed on a side of the tunneling barrier layer 704opposite the interfacial layer 702. According to an exemplaryembodiment, free layer 706 is formed from CoFeB deposited using asputtering process. This free layer 706 and the above-describedreference layer 604 which are separated by the tunneling barrier layer704 form a magnetic tunnel junction. As described above, information isstored in the tunnel junction magnetic memory cell as an orientation ofthe magnetization of the free layer 704 as compared to an orientation ofthe magnetization of the reference layer 604, which according to thepresent techniques can be switched by driving a current through themagnetic tunnel junction—see below.

Finally, to complete the magnetic memory cell, a capping layer 708 isformed on a side of the free layer 706 opposite the interfacial layer704. According to an exemplary embodiment, the capping layer 708 isformed from Fe using sputtering.

An array of the present magnetic memory cells may be employed as an MRAMdevice. See, for example, FIG. 8. FIG. 8 is a diagram illustrating amagnetic memory cell array 800. Magnetic memory cell array 800 includesbit lines 802 and word lines 804 running orthogonal to each other aboveand below a plurality of magnetic memory cells 801. Each of the magneticmemory cells 801 is representative of any of the magnetic memory cellconfigurations presented herein. For example, each magnetic memory cell801 may be configured as the magnetic memory cell illustrated in FIG. 5or as the magnetic memory cell illustrated in FIG. 7. The configurationof magnetic memory cell array 800 shown in FIG. 8 is merely exemplary,and other configurations are possible. By way of example only, magneticmemory cell array 800 can be configured to have bit lines 802 run belowmagnetic memory cells 801 and word lines 804 run above magnetic memorycells 801.

Methods for writing data to magnetic memory cell array 800 will bedescribed in detail below. In general, however, each word line 804applies a magnetic field H_(word) along a y-axis of each magnetic memorycell 801, and each bit line 802 applies a magnetic field H_(bit) alongan x-axis of each magnetic memory cell 801. The y-axis comprises a hardswitching axis of each magnetic memory cell 801 and the x-axis comprisesan easy switching axis of each magnetic memory cell 801.

FIG. 9 is a diagram illustrating exemplary methodology 900 for writingdata to a magnetic memory cell array, such as magnetic memory cell array800, described in conjunction with the description of FIG. 8, above,having a plurality of word lines oriented orthogonal to a plurality ofbit lines and a plurality of magnetic memory cells (e.g., the magneticmemory cells of FIG. 5 or the magnetic memory cells of FIG. 7, bothdescribed above) therebetween.

In step 902, a current is passed along a given one of the word lines (aword line current) thereby selecting all of the magnetic memory cells onthat given word line (i.e., the word line current destabilizes themagnetic memory cells, which essentially erases all pre-existing dataand makes the magnetic memory cells easier to write). Namely, all of themagnetic memory cells on that given word line are selected to be writtentogether at the same time. According to one exemplary embodiment, thereare 128 magnetic memory cells per word line.

In step 904, each of the magnetic memory cells selected in step 902,above, is written by sending a small current through each correspondingbit line (a bit line current). For example, if 128 magnetic memory cellsare selected in step 902 above, then a bit line current is sent througheach of the 128 corresponding bit lines to write data to those 128magnetic memory cells. The bit line current can be either a positivecurrent or a negative current. A positive current will write a logic “1”to the corresponding magnetic memory cell, and a negative current willwrite a logic “0” to the corresponding magnetic memory cell.

In step 906, the word line current is removed. In step 908, the bit linecurrent is removed. As a result, data (i.e., either a logic “1” or alogic “0”) is written to each of the magnetic memory cells selected instep 902, above.

The present techniques are further illustrated by way of reference tothe following non-limiting examples.

To determine the thickness of the Ir-containing layers used in themultilayer reference stack, the following structure was fabricated:30Ta/50Ru/30Pd/6×[2.5Co/2Pd]/5Co/tIr/2.5Co/6×[2.5Co/2Pd]/30Pdwherein the values of t=4, 4.5, 5, 5.5, 6, 7, 8, 9 Å were evaluated. Allthickness values are given in angstroms. The Ir thickness values foundto give the best antiferromagnetic coupling were t=4.5 Å and t=5 Å, seeFIGS. 10A and 10B respectively.

To evaluate the thickness for each pair of the Co-containing layers inthe multilayer reference stack, the following structure was fabricated:30Ta/50Ir/3×[t1Co/5Ir/t2Co/5Ir]/30Pdwherein the following values for t1 and t2 of the Co-containing layerswere evaluated:t1=5,t2=2.5t1=7.5,t2=5t1=10,t2=7.5t1=12.5,t2=10

All thickness values are given in angstroms. The t1 and t2 thicknessvalues of the Co-containing layers found to give the best perpendicularmagnetic anisotropy was t1=7.5, t2=5, see FIGS. 11A and 11B.

To evaluate the function of the seeding layer, the following seedinglayer configurations were tested (both at room temperature and at 250°C. sample growth temperatures):30Ta/50Ir/3×[10Co/5Ir/7.5Co/5Ir]/30Pd30Ta/50Pd/3×[10Co/5Ir/7.5Co/5Ir]/30Pd30Ta/50Ru/3×[10Co/5Ir/7.5Co/5Ir]/30Pd30Ta/50Ir/3×[10Co/5Ir/7.5Co/5Ir]/30Pd (seeding layer at 250° C.)30Ta/50Pd/3×[10Co/5Ir/7.5Co/5Ir]/30Pd (seeding layer at 250° C.)30Ta/50Ru/3×[10Co/5Ir/7.5Co/5Ir]/30Pd (seeding layer at 250° C.)All thickness values are given in angstroms. The Ir seeding layerprepared at room temperature was found to provide the best saturationfield. It is notable that the example depicted in FIGS. 11A and 11B alsoused Ir as the seeding layer and only the Co-containing layer thicknesswas varied.

Further, an evaluation of the performance of the present magnetic memorycells after an anneal at 300° C. and 360° C. was performed to determinethe stability of the reference layer. After the anneal at 300° C., theresistance area was 25.9 and the magnetoresistance was 71%. After theanneal at 360° C., the resistance area was 30.3 and themagnetoresistance was 64%. Thus, the reference layer proved to be stableeven after an anneal at these elevated temperatures.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A method of writing data to a magnetic randomaccess memory device having a plurality of word lines orientedorthogonal to a plurality of bit lines, and a plurality of magneticmemory cells configured in an array between the word lines and bitlines, the method comprising the steps of: providing a word line currentto a given one of the word lines to select all of the magnetic memorycells along the given word line, wherein at least one of the selectedmagnetic memory cells comprises: a reference magnetic layer comprisingmultiple cobalt (Co)-containing layers oriented in a stack, whereinadjacent Co-containing layers in the stack are separated by an iridium(Ir)-containing layer such that the adjacent Co-containing layers in thestack are anti-parallel coupled by the Ir-containing layer therebetween,and wherein an even number of the Co-containing layers is present in thestack such that the stack comprises a positive integer N number of pairsof the Co-containing layers; a free magnetic layer separated from thereference magnetic layer by a barrier layer, wherein a thickness of theCo-containing layers in each of the pairs depends on a location of theCo-containing layers in the stack relative to the free magnetic layersuch that in each of the pairs, the Co-containing layer located in thestack closer to the free magnetic layer is thinner than theCo-containing layer located in the stack farther away from the freemagnetic layer; providing a bit line current to each of the bit linescorresponding to the selected magnetic memory cells; removing the wordline current; and removing the bit line current.
 2. The method of claim1, further comprising the step of: providing a positive bit line currentto write a logic “1” to one or more of the selected magnetic memorycells.
 3. The method of claim 1, further comprising the step of:providing a negative bit line current to write a logic “0” to one ormore of the selected magnetic memory cells.
 4. The method of claim 1,wherein N is from 1 to
 5. 5. The method of claim 1, wherein N is
 3. 6.The method of claim 1, wherein a given one of the pairs comprises afirst Co-containing layer having a thickness t1 and a secondCo-containing layer having a thickness t2, wherein the secondCo-containing layer is located in the stack above the firstCo-containing layer, and wherein t2<t1.
 7. The method of claim 6,wherein t2 is from about 20% to about 50% less than t1.
 8. The method ofclaim 1, wherein the at least one selected magnetic memory cell furthercomprises a seeding layer adjacent to the reference magnetic layer. 9.The method of claim 8, wherein the seeding layer has a face centeredcubic (FCC) structure.
 10. The method of claim 8, wherein the seedinglayer comprises platinum (Pt), ruthenium (Ru), or Ir.
 11. The method ofclaim 8, wherein the seeding layer has a thickness of greater than about5 nm.
 12. The method of claim 8, wherein the seeding layer has athickness of from about 5 nm to about 50 nm, and ranges therebetween.13. The method of claim 1, wherein the at least one selected magneticmemory cell further comprises an interfacial layer between the referencemagnetic layer and the free magnetic layer.
 14. The method of claim 13,wherein the interfacial layer comprises cobalt-iron-boron (CoFeB) with atantalum (Ta) underlayer.
 15. The method of claim 1, wherein the freemagnetic layer comprises CoFeB.
 16. The method of claim 1, wherein 128of the magnetic memory cells are present along the given word line. 17.The method of claim 1, wherein the Ir-containing layer has a thicknessof from about 4 Å to about 9 Å, and ranges therebetween.
 18. The methodof claim 1, wherein the Ir-containing layer has a thickness of fromabout 4.5 Å to about 5 Å, and ranges therebetween.
 19. The method ofclaim 1, wherein the stack comprises an equal number of Co-containingand Ir-containing layers.
 20. The method of claim 1, wherein the stackcomprises differing numbers of Co-containing and Ir-containing layers.